THE DISTRIBUTION OF ALGJE ON ROCKS 



215 



below the fringe of green and brown which is frequently quite 

 conspicuous on rocks near low-water mark. Some of the form& 

 are found at depths of two hundred feet or more, the depth limit 

 varying with the clearness of the water. Most of the red algae 

 seem to prefer shaded situations among the rocks, and it is 

 probable that their characteristic color is associated with these 

 3 



FIG. 202. The distribution of conspicuous algae on two rocks of Spindle 

 Ledge, Woods Hole, Massachusetts, in September, 1906 



The dotted line is low-water mark, and the rocks are completely covered at high 

 tide. Blue-green algae: 1, Calothrix scopulorum, on the highest part of the 

 rock. Green algae: 2, Ulva lactuca var. rigida (sea lettuce) ; 3, Enteromorpha 

 prolifera (sea lettuce) ; both forms grow well above low-water mark. Brown 

 algae: 4, Fucusvesiculosus(TOckvfeed), in patches, butnot plentiful in thesummer; 

 5, Chordaria flagelliformis (shoestrings) , heavy growths, well below low-water 

 mark. Red algae: 6, Nemalion multifidum, on the higher parts of the rock; 

 7, Ceramium rubrum, a well-marked fringe at low-water mark; 8, Polysi- 

 phonia violacea, a well-marked fringe just below low-water mark ; 9, Chondrus 

 crispus (Irish moss) , large patches from one to three feet below low-water mark 



peculiar subdued light relations, so different from those of other 

 algae. Some types are incrusted with lime and form the curious 

 growths on rocks called corallines. 



242. The distribution of algae on rocks. Many seaweeds are 

 only found in certain situations upon rocks, where they grow in 

 patches and fringes, and frequently exhibit a sort of zonation 

 somewhat similar to the distribution of plant life around the 



